The aforesaid material and product made therefrom are to meet quite a number of requirements as for strength characteristics, corrosion-resistance in water and in high-temperature steam, hydrogenation-proofness, and resistance to radiation growth and to creep. The material should possess high processibility characteristics as being aimed at making, e.g., thin-walled pipes for fuel claddings, spacer grids, and other construction elements of the nuclear reactor core.
Most suitable for the purpose are zirconium-based materials, containing 0.5-1.5 wt. % Nb, 0.9-1.5 wt. % Sn, and 0.3-0.6 wt. % Fe, up to 0.2 wt. % Cr in combination therewith. Said materials, when compared with alloys, wherein the maximum content of the third component is within 0.28 wt. %, feature a higher corrosion resistance, including higher resistance to nodular corrosion, as well as a higher resistance to radiation growth and to creep.
Known in the art is a zirconium alloy (SU, Al, 1,751,222), consisting essentially of the following components on a weight percent basis;
Sn--0.9-1.2
Fe--0.3-0.6
Nb--0.5-1.1
Zr--being the remainder.
The aforesaid alloy features fairly high corrosion-resistance and mechanical properties: tensile strength up to 490-580 MPa (20.degree. C.) and 275-365 MPa (at 350.degree. C.), yield point up to 345-390 MPa (at 20.degree. C.) and 185-245 MPa (at 350.degree. C.) corrosion resistance when exposed to the effect of a steam-and-water mixture at 300.degree. C. irradiation, until formation of fluences 3.7 10.sup.24 m.sup.2 (3400 h) and 1 10.sup.25 m.sup.-2 (7840 h) in terms of weight gain) up to 30-40 mg/sq dm and 80-95 mg/sq dm, respectively, and a rate of creep at 350.degree. C. and under a lead of 100 MPa up to (1.3-3.6) 10.sup.-5 %/h.
The alloy in question, however, is featured by liability to form a banded arrangement of large-size particles (up to 1.5 m) of a fairly stable intermetallide enriched in iron (ZrFe.sub.3) in the ingot and at early stages of its processing, which leads during plastic working of this alloy with high reduction ratios, to origination of microcracks at the places of aggregation of such particles. To eliminate large-size particles in the alloy structure involves beta-treatment of an ingot with prolonged holding periods at high temperatures, with the resultant higher metal consumption due to an increased thickness of the gas-saturated layer being removed and hence higher cost of finished products.
Furthermore, despite the use of beta-treatment, rather large (up to 1.0 m) particles of the ferriferous intermetallides (ZrFe.sub.3 inclusive) are reestablished in the structure of finished products made of said alloy, with the result that a total density of particles in the alpha-zirconium matrix is noticeably reduced. This can be explained by a high tendency of such intermetallides to coagulation at the temperatures of recrystallization alpha-annealing between the stages of multiply repeated cold straining which are applied when making products from said alloy. Presence of large particles of intermetallides featuring a reduced distribution density in the matrix affects adversely corrosion resistance and breaking strength characteristics of the material the product is made of.
Known in the art are a zirconium alloy for making components for light-water reactors and a method for its treatment with a view to attaining a definite creep level thereof (U.S. Pat. No. 5,125,985).
Said alloy consists essentially of, on a weight percent basis;
0.5 to 2.0 Nb;
0.7 to 1.5 Sn;
0.07 to 0.28 of at least one metal selected from the group consisting of the group of elements incorporating Fe, Ni, and Cr;
up to 220 mln.sup.-1 C;
the remainder being Zr.
Use is made in said method of beta-annealing following extrusion, a number of intermediate reductions of the product cross-sectional area and of intermediate recrystallization alpha-annealings, followed by final reduction of the product cross-sectional area and its finish annealing for stress relieving. The essence of the method consists in finding an optimum combination of a degree of cross-sectional areas reduction and an intermediate annealing temperature following such a cross-sectional reduction at the stage of cold working. Beta-hardening can be reserted to at the final stage following cold working.
The process is capable of producing the products having a constant rate of creep at 385.degree. C., as well as those with a 10.degree.-190.degree. reduction of their cross-sectional area.
However, products made from said alloy suffer from an inadequately broad range of anticorrosive properties, including too low resistance to nodular corrosion in boiling water. This is mainly due to the fact that, on account of a relatively low content of the third component which is a rule Fe, there cannot be obtained a definite qualitative and quantitative composition of the second-phase particles and a required density of their distribution, which ensure in combination a high level of anticorrosive and mechanical properties, since a considerable proportion among the second-phase particles is made by particles reach mainly in Nb, whereas the ferriferous intermetallides that determine the anticorrosive and mechanical properties of the product are formed in a small amount and feature a large interparticle distance (more than 0.50 m).
Further on, when using the treatment method proposed before one cannot attain in the product a combination of high creep resistance, crack-resistance, and corrosion resistance, including alloys having a higher Fe content, i.e., from 0.3 to 0.6 wt. %. This can be explained by the fact that one cannot attain a high degree of homogeneity of the alloy matrix granular structure with the second-phase finely divided particles (under 0.1-0.2 m) uniformly distributed therein In case of mechanical treatment without beta-hardening at the stage of cold working of an ingot it is second-phase particles sized 0.1-0.4 m that are mainly present in the alloy structure, which fails to provide an adequately high corrosion resistance of the alloy. Apart from that, with such a treatment one cannot avoid the presence of sporadic large-size (up to 1.5 m) segregates of intermetallide of the ZrFe.sub.3 type which, when turning into aggregations, affect badly the toughness and crack resistance of the material. Application of beta-hardening at the final stage of cold working prevents formation of large intermetallides in the alloy structure; however, once-through cold straining and posthardening annealing are not sufficient for eliminating the "inherited" banded arrangement (lineage) of the second-phase segregates and attaining, as a whole, a homogeneous fine-grained structure featuring a steady high level of corrosion resistance and breaking strength. In addition, the material in question (especially high Fe-content alloys) has a reduced reserve of technological plasticity after beta-hardening at the final stage of cold working, this being due to hardening stresses, which prevent the use of high degrees of cold straining at the final stage of cold working and places substantial limitation upon a practicabiity of the aforesaid method to products featuring a high cross-sectional reduction ratio at the final stage of cold working.
One state-of-the-art zirconium-based material and a process for producing articles therefrom is known to be disclosed in U.S. Pat. No. 4,649,023. The material contains, on a weight percent basis: 0.5-2.0 Nb; 0.9-1.5 Sn; 0.09-0.11 of a third component selected from the group consisting of Fe, Cr, Mo, V, Cu, Ni, and W, Zr being the remainder.
The process for producing articles from said material consists of the following stages:
making an ingot and its plastic working to produce a blank or workpiece;
beta-treatment (hardening) of the blank;
initial mechanical treatment of the blank at a temperature below 650.degree. C.;
mechanical treatment be a multistage cold working at a temperature below 650.degree. C.;
annealing between the cold working stages at 500.degree.-650.degree. C.;
finish annealing at a temperature below 650.degree. C.
The process under consideration provides for a uniform distribution of the second-phase particles having an average size below 800 .ANG. in the microstructure of the material, which renders it corrosion-resistance and resistant to saturation with hydrogen in a medium of high-temperature steam in nuclear reactors.
However, density of second-phase Fe-containing particles is inadequate, due to too a low content of a third component in said material, in order to impart a higher corrosion-resistant property thereto.
Furthermore, said process is applicable only for producing articles from zirconium-based materials, containing not over 0.25 wt. % of a third component, whereas the use of said process for producing articles from said material containing a higher weight percent of a third component (0.3 to 0.6) results in a higher percent of reject for microcracks after a first cold rolling procedure with an adequately high reduction ration applicable in rolling practice. This can be explained by a reduced technological plasticity of the material due to a high level of residual hardening stresses therein resulting from beta-annealing followed by a high-rate cooling carried out prior to cold-rolling procedures.
Practising a first cold-rolling operation with low reduction ratios, for fear of cracking of the material, necessitates carrying out a great number of cold working operations with intermediate annealing operations, which makes the process much less economic. Moreover, in this case the anticorrosive and mechanical properties of the material are also impaired due to formation of a less fine-grained and less homogeneous structure in the material of the product, featuring larger segregates of the second-phase particles having an interparticle distance on the order of 0.45-0.50 m. This is concerned with the fact that in this case an intermediate annealing is conducted at a higher temperature and for a longer period of time, which leads ultimately to a growth of the grain and of the second-phase particles.